With respect to drug disposition, children (especially neonates and infants) are different than the adults. Therefore it is not appropriate to select dosing regimen for this special population based on empirical scaling (e.g., based on body weight) of the adult dose. This issue becomes more significant as it is not always possible to establish safety and efficacy of drugs in the children at clinic due to logistical, ethical, safetyand medico- legal concerns. For instance, out of 399 prescribed drugs to neonates/infants between 1997-2010, only 28 drugs were studied for the safety and/or efficacy. Thus, it is imperative that novel alternative approaches are developed to predict safe and efficacious dosing regimens for children. One such approach is to integrate age- dependent physiological parameters with drug specific parameters (e.g., in vitro enzyme/transport kinetic data) to develop a pediatric physiologically based pharmacokinetic (pPBPK) model. Once validated, such fully mechanistic pPBPK model can be generalized for any drug. However, the biggest hurdle in developing such models for children are the lack of absolute ontogeny data on the proteins that are related to drug disposition, i.e,, drug metabolizing enzymes (DMEs) and transporters. It is known that developmental pattern exists in the expression of major hepatic DMEs, but the available data are either qualitative/semi-quantitative or completely missing for most of the DMEs/transporters. Therefore, as a first step towards rectifying this gap in knowledge we propose to quantify the hepatic expression of DMEs and transporters in our unique pediatric livers (n=220) and compare this expression with that in adults. We will use selective and robust multiple reaction monitoring (MRM) proteomic approach to quantify these proteins. Once the age-dependent protein abundance data are available, these data can be integrated with in vitro kinetics and other developmental (physiological) information to construct a pPBPK models. Such rationally designed models can be validated using available clinical data on the model compounds, and then generalized to drugs that are eliminated by these mechanisms in the liver. Because mechanistic pPBPK models can delineate fractional role of individual metabolic/transport pathways in drug disposition, such mechanistic tools are also capable of accurately predicting drug-drug interactions (DDIs) and pharmacogenetic variability mediated by these pathways. Hence, this proposal addresses the mechanisms of hepatic drug disposition in neonates to adolescents. The pPBPK model generated in this study will be of enormous value with respect to child health as these will be important to assess the risk associated with the first use of drug (or other xenobiotics) in children (including neonates/infants, where the ontogeny matters most).